1. Field of the Invention
The present invention relates to an exhaust gas purification device for an engine.
2. Description of the Related Art
With respect to an engine in which a lean air-fuel mixture is burned, the same applicant has proposed a new type of engine in which a NO.sub.x absorbent is arranged in the exhaust passage of the engine. This NO.sub.x absorbent absorbs the NO.sub.x when the air-fuel ratio of the exhaust gas flowing into the NO.sub.x absorbent is lean, and this NO.sub.x absorbent releases the absorbed NO.sub.x when the air-fuel ratio of exhaust gas flowing into the NO.sub.x absorbent becomes rich. In this engine, the NO.sub.x produced when the lean air-fuel mixture is burned is absorbed by the NO.sub.x absorbent. The air-fuel ratio of the exhaust gas flowing into the NO.sub.x absorbent is temporarily made rich before the absorbing ability of the NO.sub.x absorbent is saturated, and at this time, the NO.sub.x is released from the NO.sub.x absorbent. In addition, at this time, the NO.sub.x thus released is reduced (See copending U.S. patent application Ser. No. 66,100 derived from PCT application JP92/01279).
However, since sulphur is contained in fuel and lubricating oil of the engine, sulphur oxides SO.sub.x is contained in the exhaust gas and, in the above-mentioned engine, SO.sub.x is absorbed in the NO.sub.x absorbent together with NO.sub.x. Nevertheless, even if the air-fuel ratio of the exhaust gas flowing into the NO.sub.x absorbent is made rich, SO.sub.x thus absorbed is not released from the NO.sub.x absorbent, and thus the amount of SO.sub.x stored in the NO.sub.x absorbent is gradually increased. However, if the amount of SO.sub.x stored in the NO.sub.x absorbent is increased, the amount of NO.sub.x which the NO.sub.x absorbent is able to absorb is gradually reduced, and thus a problem arises in that NO.sub.x cannot be absorbed in the NO.sub.x absorbent.